Thermally shock resistant materials having low thermal expansion find use in applications in which it is critical to maintain product dimensions, especially for example during high temperature cycling as in automotive catalytic applications.
Crystalline compounds having low thermal expansion frequently contain a high proportion of cations with high valences coordinated by a small number of oxygen anions. Thus, low expansions are more likely to be associated with substances with a large fraction of tetrahedrally coordinated cations having valences of .gtoreq.4+, a lesser fraction of octahedrally coordinated cations with valences of .gtoreq.3+, and a minimum of highly coordinated (eight or more oxygens) cations with valences of .ltoreq.2+. Some silicophosphates having the general formula AM.sup.3+ M.sub.2.sup.4+ Si.sub.2 P.sub.6 O.sub.25, where A=K, Rb, Tl, Cs, M.sup.3+ =Mo or Ti, and M.sup.4+ =Mo, Ti, or Sn, fall into this category. Such are described in an article entitled "Silicophosphates with an intersecting tunnel structure: AM.sub.3 P.sub.6 Si.sub.2 O.sub.25 and AMo.sub.3 P.sub.5.8 Si.sub.2 O.sub.25 ", Materials Chemistry and Physics, 12 (1985) 537-543. There is a high fraction of tetrahedrally coordinated Si.sup.4+ and P.sup.5+, an intermediate fraction of octahedrally coordinated M.sup.3+ and M.sup.4+ and a small fraction of highly coordinated A.sup.1+. Because of the unusually low valences of the Mo and Ti cations in these Mo, Ti, and Sn based materials, (stable valence of Mo is 6+, and stable valence of Ti is 4+, in air), they must be synthesized in glass ampoules from a stoichiometric mixture of the oxides plus metal, and are unstable in air. Thus, their potential application is severely limited.
It would be advantageous to have silicophosphates that are stable in air and especially advantageous to have such materials have low thermal expansion at high temperatures.
The present invention provides such materials.